1. Technical Field
Embodiments of the present invention relate to a document reading unit, an image forming apparatus including the document reading unit, and a method of detecting an original document using with the document reading unit to cause an image sensor to read an image on an original document.
2. Related Art
As an example of a document reading unit, Japanese Patent Application Publication No. JP 2005-354205-A discloses a document reading unit that has a configuration provided with a platen cover that opens and closes with respect to a contact glass and an optical table that includes an LED array including multiple LED elements aligned in a main scanning direction of an original document along the surface of the contact glass. The optical table moves in a sub scanning direction that is a direction along the surface of the contact glass and is perpendicular to the main scanning direction so as to optically scan an image formed on the original document placed on the contact glass. Further, when the platen cover is closed with respect to the contact glass, the original document is pressed against the contact glass. In this state, when a read start instruction is issued, the document reading unit moves the optical table in the sub scanning direction while irradiating respective light beams emitted by the multiple LED elements of the optical table functioning as a moving light emitting part toward the original document and obtaining reflected light beams reflected on the original document. Consequently, the obtained reflected light beams are received by CCDs functioning as image sensors to read the image of the original document.
Prior to reading the image of the original document as described above, a controller of the document reading unit performs a length specifying operation to specify the length in the main scanning direction of the original document placed on the contact glass. To perform the length specifying operation, the document reading unit includes an open/close sensor in the vicinity of the platen cover to detect an opening/closing operation of the platen cover. When a user opens the platen cover to place the original document on the contact glass, the open/close sensor detects the opening/closing of the platen cover. When the opening of the platen cover is detected, the controller of the document reading unit causes the optical table to move from a given home position in the sub scanning direction to the document size detection position that located opposite the leading area of the original document. Thereafter, when the open/close sensor detects start of the closing operation of the platen cover, the controller of the document reading unit repeats the following operations until the length in the main scanning direction of the original document is specified. That is, the controller causes any of multiple LED elements included in the LED array to turn on, so that the CCDs receive the reflected light. Then, based on the amount of the reflected light, the controller determines whether the original document is present or not over the lighting LED elements. By sequentially changing the LED elements to be lit, the controller performs determination of presence of the original document on the contact glass. Then, based on the determination results, the controller specifies the length in the main scanning direction of the original document.
In JP 2005-354205-A, the document reading unit having the above-described configuration can decrease disturbances due to the LED light emitted by the LED array coming into user's eye(s) during the length specifying operation.
JP 2005-354205-A is believed to have disclosed the following operations to decrease disturbances. As the platen cover is being closed with the original document placed on the contact glass, the document reading unit starts the length specifying operation and lights the LED elements. Thereafter, when the platen cover is completely closed, even in the region in which no original document resides, the surface of the platen cover that presses the original document reflects the LED lights well even in the region that no original document is located. Therefore, the controller misdetects that the original document resides in the region. Due to this reason, the length specifying operation is required to start when the user starts to close the platen cover and to finish before when the platen cover is completely closed. During the period, since the platen cover is not fully closed, the LED light can easily come into the user's eye(s) from an open gap between the contact glass and the platen cover.
In the document reading unit disclosed in JP 2005-354205-A, the LED elements are turned on in a given order during the length specifying operation. By so doing, the amount of light of the LED elements can be reduced when compared with the whole LED elements are turned on simultaneously. Accordingly, the amount of light coining into the user's eye(s) at the same time from the open gap between the contact glass and the platen cover may be reduced, thereby decreasing the user's disturbances.
However, if a relatively large solid image is formed on an upstream leading area (hereinafter, simply a “leading area”) in the sub scanning direction of the original document, it is likely that the document reading unit performs false detection in length of the original document in the main scanning direction thereof. Specifically, the optical table located at a document size detection position is disposed facing the leading area of the original document. Some original documents do not have margins in the leading area thereof while typical original documents mostly have. If a relatively large solid image is formed in the leading area of the original document, reflection of light is blocked by the solid image in a region in which the solid image is formed in the main scanning direction of the original document. Therefore, an amount of reflection of light on the leading area of the original document is significantly low when compared with the amount of reflection of light on the pure surface of the original document. Consequently, the region having the solid image thereon is misdetected as a region that does not reside on the original document. Accordingly, the length in the main scanning direction of the original document is likely to be misdetected.